A Modest Analysis:

"In a system...where the entire continuity of the...process rests upon credit, a crisis must obviously occur -- a tremendous rush for means of payment -- when credit suddenly ceases and only cash payments have validity. At first glance, therefore, the whole crisis seems to be merely a credit and money crisis. And in fact it is only a question of the convertibility of bills of exchange into money. But the
majority of these bills represent actual sales and purchases, whose extension far beyond the needs of society is, after all, the basis of the whole crisis. At the same time, an enormous quantity of these bills of exchange represents plain swindle, which now reaches the light of day and collapses; furthermore, unsuccessful speculation with the capital of other people; finally, commodity-capital which has depreciated or is completely unsaleable, or returns that can never more be realized again. The entire artificial system of forced expansion of the economy cannot, of course, be remedied by having some bank, like the Bank of England, give to all the swindlers the deficient capital by means of its paper and having it buy up all the depreciated commodities at their old nominal values. Incidentally, everything here appears distorted, since in this paper world, the real price and its real basis appear nowhere, but only bullion, metal coin, notes, bills
of exchange, securities. Particularly in centers where the entire money business of the country is concentrated, like London ...the entire process becomes incomprehensible."

-- Karl Marx's Capital, Volume 3, Chapter 30, "Money-Capital and Real Capital", 1867.